Adipokinetic hormones (ADK) of insects form, together with the red pigment concentrating hormone of crustacea, an extended peptide family. Members of this family have been found in all insects examined, and more than twenty different peptides from different animals have been isolated. Peptides of the AKH family control mobilization of energy stores in times of need, e.g. from lipids for flight, from fat body lipids and carbohydrates in molting larvae. The corpora cardiaca organs contain neurosecretory cells that have been shown to synthesize and store AKH. The response of the fat body to liberated AKH varies during development, inducing strong lipid mobilization in adult moths, but only small amounts during the larval stage. The localization and characterization of the AKH receptor is therefore of prime interest for the basic understanding of this hormone, as well as for practical possibilities for blocking the hormone action. Very few receptors of insect peptide hormones have been characterized. Tritium labelled Manduca sexta adipokinetic hormone was synthesized (specific activity 27 Ci/mmol) by dehalogenation of p-iodo-Phe-MAKH using tritium gas, and was found to be fully active in a binding assay. Membrane fractions prepared from fat body yielded a Kd of 7 x 10-10M and receptor concentration was estimated to be ca. 0.5 pmol/mg membrane protein. No binding was found from membrane fractions prepared from brain or heart or flight muscle of M sexta, or from the fat body of cockroach. Specific binding (although low, 15% of total) was also observed in membrane preparations from the pterothoracic ganglion of M sexta. Further experiments are under way to characterize the MAKH binding site.